The present invention relates to a wear resistant slide member made of iron-base sintered alloy coming into slide contact with one another under a large surface pressure.
In a valve moving mechanism of an internal combustion engine, members such as a rocket arm (its anti-cam slide surface portion) held in slide contact with a cam, a valve lifter, and the like are required to have excellent wear resistance called abrasion-resistance hereinafter.
In the prior art, these members were provided as members made of forged steel, cast steel, alloyed cast iron and the like with their contact surfaces with a cam subjected to surface hardening by heat treatment, carburizing and quenching treatment, chill hardening, hard-Cr-plating, etc.
However, carburized and quenched members are poor in anti-scuffing property (note: "scuffing" means that phenomenon that coagulation between frictional surfaces is remarkable and thereby surface nature becomes rough), chill-hardened members are poor in durability, hard-Cr-plated members may generate peeling caused by local contact or abrasive peeling, and thus the known members respectively involve problems.
With regard to a cam shaft, heretofore, cam shafts made of cast iron (for example, made of JIS FC 25.about.30 material or made of alloyed cast iron) have been widely used, and the procedures have been employed such that upon casting, a chilled layer is formed along a cam surface by making use of a chilling block, or that after casting, a slide contact surface portion forming a cam actuating surface is subjected to remelt-hardening treatment (after molten by flame or by high-frequency heating, cooled by itself) and thereby a chilled layer is formed.
Recently, however, due to the fact that accompanying realization of high speed and high output power of internal combustion engines, a slide surface pressure on a cam surface is increased and hence high abrasion-resistance as compared to the prior art is required, even if a chilled layer is formed along a cam surface portion of a cam shaft upon casting, sufficient abrasion resistance cannot be obtained, the abrasion resistance cannot be said to be fully satisfactory.
On the other hand, with regard to a rocker arm held in slide contact with a cam, carburized and quenched members are poor in anti-scuffing property, chill-hardened members are poor in durability, and hard-Cr-plated members may generate peeling caused by local contact or abrasive peeling, as stated above.
Moreover, abrasion accompanying slide contact between a cam and a rocker arm depends upon a combination of materials of the respective members, and how to select the combination is a difficult problem. As a member satisfying this requirement, anti-cam slide members formed of iron-base sintered alloys have been proposed, and in these alloys various factors such as size, amount (field-of-view occupation area proportion), etc. of carbides dispersed in a base phase (matrix phase) and influencing hardness are defined, and improvements in abrasion-resistance and anti-scuffing property were contemplated while suppressing abrasion of a cam (for instance, refer to Japanese Patent Publication No. 59-7003 (1984), Laid-Open Japanese Patent Specification No. 60-63350 (1985) and Laid-Open Japanese Patent specification No. 60-155650 (1985)).
However, according to the understanding of various factors with respect to carbides in the iron-base sintered alloys in the prior art, the construction of the entire metallurgical structure is not clear, and so, it is not always possible to attain the desired object.